Development, fabrication and testing of the scanning and calibration subsystems for the Tropospheric Water and Cloud ICE instrument for 6U CubeSats
Date
2019
Authors
Kilmer, Braxton, author
Reising, Steven C., advisor
Chandrasekar, V., committee member
Chiu, Christine, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Global observations of ice cloud particle size and ice water content are needed to improve weather forecasting and climate prediction. The interaction between ice particles and upwelling radiation at sub-millimeter-wavelengths strongly depends on ice particle size and observation frequency. Sub-millimeter-wavelength radiometry provides the capability to fill an observational gap by allowing the detection and sizing of ice particles with diameters between 50 μm and 1 mm. Atmospheric temperature and water vapor profiles can also be yielded at sub-millimeter-wavelengths. The Tropospheric Water and Cloud ICE (TWICE) millimeter- and sub-millimeter-wave radiometer instrument is currently under development for 6U CubeSats in a joint effort among Colorado State University (lead), NASA/Caltech Jet Propulsion Laboratory, and Northrop Grumman Corporation. The TWICE radiometer instrument is designed to provide global measurements of cloud ice, as well as temperature and water vapor profiles in the upper troposphere/lower stratosphere. The TWICE radiometer instrument has 16 frequency channels near 118 GHz for temperature profiling, near 183 and 380 GHz for water vapor profiling, and centered on 240, 310, 670, and 850 GHz quasi-window channels for ice particle sizing. The TWICE radiometer instrument uses a conical scanning strategy to observe the Earth's atmosphere and surface. The complete TWICE scan is designed to sweep out a 200° arc once per second, and the scan direction reverses every second interval. The TWICE scanning system is designed to fit inside a 6U CubeSat in terms of volume and mass, while meeting the torque and acceleration requirements of the scanning radiometer instrument. A stepper motor and gearbox mechanism were selected for the TWICE scanning system. Precisely placed position sensors, in combination with stepper motor step calculation, provide sufficient angular position data, in place of a traditional encoder. The TWICE scanning system has been tested, and angular position analysis has been performed. The TWICE instrument performs end-to-end, two-point radiometric calibration by observing an ambient temperature calibration target and cosmic microwave background reflector during each conical scan. The ambient calibration target is designed to enable simultaneous blackbody measurements at all TWICE millimeter- and sub-millimeter-wave channels. Calibration target design parameters, including size, geometry, thermal and electromagnetic properties, have been chosen to meet the performance requirements of the ambient target and to minimize temperature gradients. Reflection coefficient measurements have been performed in the millimeter to sub-millimeter wavelength range of the TWICE channels. Thermal analysis of the ambient calibration target has been performed using ANSYS software. The resulting ambient calibration target design meets functional requirements as well as size and weight constraints to fit into a 6U CubeSat. The TWICE radiometer instrument employs several subsystems that need to communicate during nominal operation. An interface board was designed to meet the communication needs of and provide power regulation for the various interfacing subsystems of the instrument. The interface board is responsible for controlling the scanning subsystem of the radiometer instrument, performing temperature data acquisition for the radiometer instrument front end and the ambient calibration target, routing signals to and from the control and data handling subsystem of the radiometer instrument, and regulating power to the on-board computer. The interface board has been manufactured and its performance has been tested.